BACKGROUND: The monoclonal antibody natalizumab is a novel therapeutic option in the treatment of relapsing forms of multiple sclerosis. In general, therapy with natalizumab is well tolerated. Allergic reactions and acute infusion reactions typically occur during or shortly after infusion, with a peak at the second infusion. Delayed infusion reactions resembling serum sickness-type reactions (type III reaction) are commonly reported in other monoclonal antibody therapies (eg, infliximab and rituximab), but are not described yet for natalizumab. RESULTS: Delayed infusion reactions occurred in 4 of 40 relapse-remitting multiple sclerosis patients treated with natalizumab. CONCLUSIONS: Clinicians need to consider the occurrence of infusion reactions, with especially delayed reactions occurring more frequently than previously assumed. Our cases illustrate that some of these infusion reactions may be treated effectively with steroids and reduction of the infusion rate. In cases of antibody-mediated reactions, treatment should be stopped immediately.
